Switching devices are used for interrupting a current or protecting an electric circuit in the event of an electrical failure for example due to a short circuit. A Switching device may comprise contacts including a stationary and a movable contact, which during a normal operation are in mechanical and electrical connection. When the contacts are separated from each other a current breaking operation is effected. In addition to separating the contacts, a current breaking/interrupting operation involves extinguishing an arc between the contacts, and to force the current to decrease to zero.
When breaking a current without any natural zero-crossings, it is necessary to force the current down to zero. One common practice is to create a voltage across the breaking point that is higher than the system voltage thus forcing the current to decrease to zero. In order to achieve such a voltage across the breaking point it is desired to stretch the breaking arc over a long distance since the length of the arc increases the arc voltage and a long arc is also easily cooled and split into several shorter arcs that further increase the arc voltage.
An arc may be prolonged by either separating the contacts to a desired length so that the arc is stretched out or enabling the originally short arc to move along a path that stretches the short arc. When the contacts separate from each other at a limited distance, the arc must leave contact points quickly to avoid erosion of contact materials. Thus, an arc extinguishing chamber/chute is provided so that the arc moves away from the contacts into it, which further increases the arc voltage. It is desired that the arc moves along the desired path in a correct direction and with sufficient speed so that the arc voltage increases to a sufficient high value to break the current.
It is also known in the art that a so-called parallel make-and-break contact system may be used, wherein the system comprises a main contact assembly including a movable main contact and a stationary main contact and an arcing contact assembly including a movable arcing contact and a stationary arcing contact coupled in parallel with the main contact assembly. Different characteristics that are required for different modes of operations are thus optimized. In such a system, the main contacts normally only conduct the current and is not involved in the switching operations that create arcs. The material in the main contact is optimized for a good conductivity thus reducing the generated power when current is flowing. On the other hand, the arcing contacts are arranged to handle switching operations and are not meant for continuously conducting the current.
EP2037472A2 describes a switching system including main contacts and arcing contacts, wherein during a switching operation there is a timing between the main contacts and the arcing contacts so that all switching (opening or closing) are handled by the arcing contacts while the main contacts conducts an electric current when the circuit is closed and are not damaged by any switching operations.